1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to satellite digital audio service (SDARS) receiver architectures.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Satellite radio operators will soon provide digital quality radio broadcast services covering the entire continental United States. These services intend to offer approximately 100 channels, of which nearly 50 channels will provide music with the remaining stations offering news, sports, talk and data channels. According to C. E. Unterberg, Towbin, satellite radio has the capability to revolutionize the radio industry, in the same manner that cable and satellite television revolutionized the television industry.
Satellite radio has the ability to improve terrestrial radio""s potential by offering a better audio quality, greater coverage and fewer commercials. Accordingly, in October of 1997, the Federal Communications Commission (FCC) granted two national satellite radio broadcast licenses. The FCC allocated 25 megahertz (MHz) of the electromagnetic spectrum for satellite digital broadcasting, 12.5 MHz of which are owned by CD Radio and 12.5 MHz of which are owned by the assignee of the present application xe2x80x9cXM Satellite Radio Inc.xe2x80x9d. The FCC further mandated the development of interoperable receivers for satellite radio reception, i.e. receivers capable of processing signals from either CD Radio or XM Radio broadcasts. The system plan for each licensee presently includes transmission of substantially the same program content from two or more geosynchronous or geostationary satellites to both mobile and fixed receivers on the ground. In urban canyons and other high population density areas with limited line-of-sight (LOS) satellite coverage, terrestrial repeaters will broadcast the same program content in order to improve coverage reliability. Some mobile receivers will be capable of simultaneously receiving signals from two satellites and one terrestrial repeater for combined spatial, frequency and time diversity, which provides significant mitigation against multipath and blockage of the satellite signals. In accordance with XM Radio""s unique scheme, the 12.5 MHz band will be split into 6 slots. Four slots will be used for satellite transmission. The remaining two slots will be used for terrestrial re-enforcement.
In accordance with the XM frequency plan, each of two geostationary Hughes 702 satellites will transmit identical or at least similar program content. The signals transmitted with QPSK modulation from each satellite (hereinafter satellite1 and satellite2) will be time interleaved to lower the short-term time correlation and to maximize the robustness of the signal. For reliable reception, the LOS signals transmitted from satellite1 are received, reformatted to Multi-Carrier Modulation (MCM) and rebroadcast by non-line-of-sight (NLOS) terrestrial repeaters. The assigned 12.5 MHz bandwidth (hereinafter the xe2x80x9cXMxe2x80x9d band) is partitioned into two equal ensembles or program groups A and B. The use of two ensembles allows 4096 Mbits/s of total user data to be distributed across the available bandwidth. Each ensemble will be transmitted by each satellite on a separate radio frequency (RF) carrier. Each RF carrier supports up to 50 channels of music or data in Time Division Multiplex (TDM) format. With terrestrial repeaters transmitting an A and a B signal, six total slots are provided, each slot being centered at a different RF carrier frequency. The use of two ensembles also allows for the implementation of a novel frequency plan which affords improved isolation between the satellite signals and the terrestrial signal when the receiver is located near the terrestrial repeater.
In any event, with different content being provided on each ensemble and inasmuch as data will be transmitted along with music content on one or both ensembles, it is conceivable that a listener will may want to access content on both ensembles simultaneously.
Unfortunately, there was no efficient satellite radio receiver architecture capable of receiving two ensembles simultaneously. Accordingly, system designers were forced to consider either replicating the data on both ensembles or replicating the tuner within the receiver. Both approaches were unacceptably costly. As a result, there was a need in the art for satellite radio receiver architecture capable of receiving two ensembles simultaneously which will not require a replication of the tuner nor a replication of the data broadcast channel on both ensembles.
The need in the art for a satellite radio receiver architecture capable of receiving two ensembles simultaneously is addressed by the invention disclosed and claimed in U.S. patent application Ser. No. 09/318,296, filed May 25, 1999 by P. Marko et al., entitled LOW COST INTEROPERABLE SATELLITE DIGITAL AUDIO RADIO SERVICE (SDARS) RECEIVER ARCHITECTURE (Atty. Docket No. XM 0006), assigned to the present assignee, the teachings of which are incorporated herein by reference.
The receiver architecture of the referenced patent involves an analog mixing of RF signals to complex baseband for digital conversion. However, as is appreciated by those skilled in the art, the analog mixing of RF signals to complex baseband for digital conversion has inherent limitations related to the dynamic range of the input signals. In practice, these limitations often steer the receiver designer to digital conversion at an intermediate frequency at the expense of higher cost and size.
One such limitation of mixing analog signals to baseband is second order intermodulation products generated in the baseband mixers and post mixer amplifiers. These undesired products develop when two RF (or IF) signal components (f1 and f2) present at the mixer input self-mix and the difference product (f1-f2) falls at baseband. If the amplitude of the difference product is sufficiently large, destructive interference with the desired baseband signal occurs.
A second limitation of analog mixing of RF signals to baseband is due to the fact that the conversion of RF signals to baseband using analog conversion results in the creation of images about 0 Hz axis due to gain and/or phase imbalance in the I and Q complex signal paths. The imbalance may be due to many causes including imperfect device matching, layout asymmetries, mechanical and process variations in present production RF circuit technology. Best case I/Q matching with standard bipolar integrated circuit processing results in a minimum image attenuation in the range of 30-40 dB. The image of the large amplitude signal creates destructive interference for the small signal. Those skilled in the art appreciate that a receiver operating in a typical land mobile environment will encounter substantially large signal amplitude variations due to the varied proximity to terrestrial transmitters.
Hence, there is a further need in the art for a receiver architecture for multiple signal reception which includes an analog conversion to baseband stage with image rejection capability effective to yield acceptable interference protection.
The need in the art is addressed by the system and method of the present invention. In general, the inventive system includes a receiver adapted to receive a signal having at least first and second carrier frequencies on which first and second information signals are modulated, respectively. The inventive receiver further includes circuitry for converting the received signal to a complex baseband signal.
In the illustrative embodiment, the received signal includes first and second ensembles. The first ensemble includes a first signal from a first source, a first signal from a second source and a first signal from a third source. The second ensemble includes a second signal from the first source, a second signal from the second source and a second signal from the third source. The receiver is adapted to selectively output the first and/or the second ensemble. Conversion of the band is achieved with quad mixers. The outputs of the mixers are digitized and selectively provided as the first and/or the second ensemble by a digital translation stage.